In recent years, control circuitry for use with induction motors have been developed, the control circuitry functioning to improve the power factor of the motor, and thus reduce the power dissipation of such induction motors when partially loaded. Typically, induction motors, when operating below full load, exhibit power factors as low as 0.1 or 0.2. As a result, relatively large currents flow, while very little work is performed. Thus, power losses occur at all points (including the motor windings) in such a distribution system, even though no mechanical power is delivered.
Based on the latter realization, recent efforts have been directed to the development of an electronic control system which raises the power factor of the system so as to provide significant energy savings as a result. Such a control system is disclosed, for example, in U.S. Pat. No. 4,052,648 to Nola, and in a corresponding article entitled "Circuit Saves Power In AC Induction Motors", by Frank J. Nola, National Aeronautics and Space Administration, Published in EDN Magazine (Sept. 5, 1979), pages 185-189, as well as in NASA Tech Brief No. NTN-78/0252 (MFS-23389), entitled "Save Power In AC Induction Motors".
More specifically, the latter discloses a power controller which reduces power losses by sensing the phase lag between voltage and current. It provides this information to circuitry which forces a motor to run at a constant, predetermined optimum power factor, regardless of load or line-voltage variations (within the motors limits). More specifically, when the load is reduced, a solid-state switch (triac) in the controller reduces the applied voltage, minimizing wasted power. As the load increases, it, in turn, increases the voltage to the proper operating level.
A significant disadvantage of such a controller resides in the fact that, whereas single-phase motors require no modification to use the controller, use of the controller with multi-phase motors (for example, the wye-connected three-phase motor) requires opening the motor and making internal connections within the motor (for example, connecting to the wye internal to the motor or to the neutral of the three-phase line), and then placing a triac with its firing circuitry in series with each phase of the motor. Another disadvantage of such prior art controllers resides in the fact that, when such controllers are first started, substantial power is wasted. That is to say, upon starting the controller, typically, maximum "in rush" current is applied, much of which current is wasted until the motor controller arrives at its normal, stabilized level of operation.
Motor controllers of the prior art have also been equipped with an "overload trip" capability, whereby, when the controller senses an overloading of the motor, operation of the motor is immediately interrupted or "tripped". However, this technique has been implemented in terms of a total and immediate trip, without regard to the amount of overload. It has been recognized that such constitutes an inefficient way of operating, since it is often not necessary to immediately interrupt the operation of the motor. For example, while a very high overloading of the motor renders it necessary to turn the motor off as quickly as possible, with slight overloading of the motor, it is possible to turn the motor off somewhat more slowly with no risk of damage. In other words, the technique of instantaneously turning off the motor upon detection of any overload, no matter how small, amounts to an inefficient and unnecessarily inconvenient method of operation.
Motor controllers having an overload trip capability typically face the problem of disabling that capability on "start up". That is, since the load applied to the motor at "start up" is typically large enough to exceed the overload threshold, inadvertent operation of the overload trip function will result in "start up" unless the function is disengaged. Thus, substantial operational improvement can be gained from providing a motor controller with the capability of automatically disabling the overload trip capability or feature for a certain period of time following "start up".
In the prior art, motors have been susceptible to damage if phase has been lost during their operation. That is to say, if phase is lost, a multi-phase motor will typically try to go to a single phase mode of operation, and will burn up. Thus, it is considered quite advantageous to have a motor controller which, upon detection of phase loss, immediately shuts the motor off.
Whereas the prior art has included implementation of a motor controller by means of silicon-controlled rectifiers (see, for example, the aforementioned patent and article of Frank F. Nola), unless such silicon-controlled rectifiers are controlled properly, so as to cut off quickly and act like fuses, the motor controllers must also include fuses. Thus, a foremost deficiency of prior art controllers resides in the fact that, even though such silicon-controlled rectifiers have been utilized, they have not been properly controlled (that is, properly pulsed) so as to act like fuses. If such were the case, significant savings in cost of the controller would be achieved.
Finally, motor controllers of the prior art have not been capable of functioning uniformly, that is to say, functioning as well in the presence of distorted AC line voltages as they function in the absence thereof.